In recent years, from the viewpoint of achieving weight reduction, thickness reduction, and downsizing of a bushing, commonality of the bushing type, simplification of the operation process and the like, polymer bushings having a solid insulation structure (complete dry type) in which a polymer covering formed of a silicone rubber or the like is directly molded on the outer peripheral surface of an insulating tube formed of an epoxy resin or the like are practically used (for example, PTLS 1 to 4).
In a polymer bushing, corona discharge is generated at the exterior surface of the polymer bushing when the electric field strengthens. In the case where corona discharge is generated for a long period of time, the polymer covering can possibly be chemically eroded, and degraded (erosion). Normally, for the purpose of preventing corona discharge, the polymer covering includes a body part that covers the outer periphery of the insulating tube, and umbrella-shaped sheds formed with a space therebetween in the longitudinal direction on the outer periphery of the body part.
In addition, in the polymer bushing disclosed in PTL 1, an electric-field stress-control layer is disposed at the interface between an insulating tube (insulator) and a polymer covering for the purpose of further improving the corona resistance. Further, the electric-field stress-control layer includes a tapered part whose thickness increases toward the front end side while the inner diameter decreases toward the front end side, and is formed in an arc surface shape. In particular, the configuration in which the front end part of the electric-field stress-control layer where the electric field concentrates is disposed at a position adjacent to a thick part of the shed is very useful for reducing corona discharge. In the organic composite porcelain bushing (polymer bushing) disclosed in PTL 2, the outer diameter of the body part of an exterior cover (polymer covering) and an umbrella part (shed) is greater than that of other portions in a region around an upper end part (a portion where a maximum electric field is generated at the surface) of an inner electrode (inner conductor).